1. Field of the Invention
The present invention relates to a top-emission organic light-emitting display device in which the source or drain electrode of a thin-film transistor is in direct contact with a transparent electrode and a reflective layer is formed below the transparent electrode to be spaced from the source electrode and drain electrode.
2. Description of the Related Art
Typically, an organic light-emitting display (OLED) device has advantages over other flat panel displays in that it has a broader range of operable temperature, is stronger against impact or vibration, has a wider viewing angle, and has a faster response speed to provide clear moving pictures. Accordingly, the organic light-emitting display device attracts attention as a next-generation flat panel display device.
The organic light-emitting display device includes an anode, an organic emission layer positioned on the anode, and a cathode positioned on the organic emission layer. In the organic light-emitting display device, when a voltage is applied between the anode and the cathode, holes are injected from the anode into the organic emission layer and electrons are injected from the cathode into the organic emission layer. The holes and the electrons, which are injected into the organic emission layer, are combined in the organic emission layer to create excitons. These excitons emit light while transitioning from an excitation state to a base state.
This organic light-emitting display device may be classified according to the position of a reflective layer. For example, in a bottom-emission organic light-emitting display device the light generated by the above-stated phenomenon is emitted downward from the substrate, and in a top-emission organic light-emitting display device the light is emitted upward from the substrate. Further, it may be further classified according to a driving manner where a passive matrix organic light-emitting display device needs a separate driving source and an active matrix organic light-emitting display device uses thin-film transistors as the active elements.
FIG. 1 is a cross-sectional view of a conventional top-emission active matrix organic light-emitting display device. Referring to FIG. 1, in the top-emission organic light-emitting display device, a thin-film transistor having a semiconductor layer 13, a gate electrode 15, a source electrode 18-1, and a drain electrode 18-2 are formed in a non-emission region on a substrate 10 by semiconductor processes. An organic light-emitting diode having a first electrode 21, an organic functional layer 23 and a second electrode 24 are formed in an emission region. A gate insulating layer 14, an interlayer insulating layer 16, a passivation layer 19 and a pixel defining layer 22 are further formed to insulate the conductive layers from one another.
Here, the first electrode 21 is a reflective electrode, and is preferably a conductive layer with an optical reflective property and a proper work function. However, because there is no proper single material that simultaneously meets these properties up to now, it is common to make the reflective electrode as a multilayer structure in which an aluminum layer 21-1 having excellent reflective efficiency is formed and an indium tin oxide (ITO) layer 21-2 having a high work function is formed thereon.
FIG. 2 is an enlarged cross-sectional view of the region A of the FIG. 1.
Referring to FIG. 2, when a reflective electrode employs a multilayer structure as described above, galvanic corrosion may occur between the aluminum layer 21-1 and the ITO layer 21-2 upon using an etching solution to pattern the reflective electrode. Moreover, the galvanic corrosion may diffuse along an interlayer interface between the aluminum layer 21-1 and the ITO layer 21-2.
Further, an aluminum oxide layer may be formed due to the interaction of aluminum and ITO between the aluminum layer 21-1 and the ITO layer 21-2. The aluminum oxide layer increases resistance between the drain electrode 18-2 and the ITO layer 21-2, such that the contact resistance between the first electrode 21 and the drain electrode 18-2 increases and dispersion of the contact resistance in the substrate increases. The increase in the dispersion of the contact resistance in the substrate causes non-uniformity in brightness between pixels when operating the top-emission organic light-emitting display device, thereby greatly degrading the quality of a screen.